One example of a spark plug that generates plasma is disclosed in Japanese Patent Application Laid-Open No. 2009-512172. The spark plug disclosed in Japanese Patent Application Laid-Open No. 2009-512172 includes a center electrode, a metal shell that surrounds at least a part of the center electrode, and an insulator provided between the metal shell and the center electrode. The insulator includes a front end portion having an outer diameter larger than an inner diameter of the metal shell. The front end portion projects from a front end of the metal shell, and a front end of the center electrode projects from the front end portion of the insulator. When voltage is applied to the center electrode of the spark plug, gas in the vicinity of the front end of the center electrode undergoes dielectric breakdown and gaseous discharge occurs, so that plasma in which gas is ionized is generated.